Some types of conventional object detection methods are designed to detect a target object from a plurality of images picked up by a pickup device, such as a camera, using pattern-matching techniques.
For example, Japanese Patent Application Publication No. H09-081726 discloses such an object detection method. The method disclosed in the Patent Publication matches the pattern of an input image in which a target object can exist with a predetermined target pattern corresponding to the target object. This matching extracts, as a matching candidate pixel, a pixel in the input image with a matching rate that exceeds a preset matching rate.
The method also obtains the distribution profile of the matching rates of pixels in the input image around the matching candidate pixel, and determines whether the distribution profile is in agreement with the profile of an ideal distribution; this ideal distribution approximates a normal distribution.
Upon determining that the distribution profile is in agreement with the profile of the ideal normal distribution, the method determines that the candidate pixel is a matching point (matching area) in the input image with respect to the predetermined target pattern.
Otherwise, upon determining that the distribution profile is wider than the profile of the ideal normal distribution, the method determines that the candidate pixel is not a matching point in the input image with respect to the predetermined target pattern.
However, the method disclosed in the Patent Publication may deteriorate the detection accuracy for target objects.
Specifically, when a target object is present in an area (referred to as “target area”) of an input image, the target area has a higher degree of similarity with respect to the target pattern, and a first different area around the target area has a higher degree of similarity with respect to the target pattern. The first different area around the target area has a size larger or smaller than that of the target area and/or has a location different from that of the target area.
In contrast, even if a part of a second different area of the input image in which no target object is present has a higher degree of similarity with respect to the target pattern, most of the remaining part of the second different area has a lower degree of similarity with respect to the target pattern.
For this reason, the distribution profile of the matching rate in the second different area in which no target object is present is not wider than that of the matching rate in the target area in which the target object is present. Thus, the method disclosed in the Patent Publication may not detect that the second different area in which no target object is present is not a matching area in the input image with respect to the predetermined target pattern. This may result in deteriorating the detection accuracy for target objects.
In addition, if the distribution profile of the target area in which the target object is present is different from the profile of an ideal normal distribution, the method disclosed in the Patent Publication may not detect that the target area in which the target object is present is a matching area in the input image with respect to the predetermined target pattern. This also may result in deteriorating the detection accuracy for target objects.